[libcxx] [llvm] [libc++][CI] Update to Clang-20 (PR #108761)

[Robin Caloudis via llvm-commits]

================
@@ -25,8 +25,7 @@ constexpr bool test_signed() {
   constexpr auto minVal = std::numeric_limits<IntegerT>::min();
   constexpr auto maxVal = std::numeric_limits<IntegerT>::max();
 
-  // TODO(LLVM-20) remove [[maybe_unused]]  since all supported compilers support "Placeholder variables with no name"
-  [[maybe_unused]] std::same_as<IntegerT> decltype(auto) _ = std::add_sat(minVal, maxVal);
+  std::same_as<IntegerT> decltype(auto) _ = std::add_sat(minVal, maxVal);
----------------
robincaloudis wrote:

Thanks for the heads up! Seems like you are already one step ahead of me. Let's limit this PR only to the update of the CI config and bring your changes in. Afterwards, I will create a PR including adjustments for libc++ tests and implementations that you have not covered. 

https://github.com/llvm/llvm-project/pull/108761